Most likely you are hearing tree frogs, but you may never see them because they blend in perfectly with the bark. Also, they tend to get quiet when you get near.

You must be right, because they are still carrying on and it's way after dark here. Loud little buggers.

LOL! Yes tree frogs are incredibly loud! I had about 5-6 singing to each other yesterday evening in the back yard and I could hardly hear my family talking to me while out on the deck! They are really hard to find too.
I have one that retires to a little ledge on the top of my shed every year.....same spot. He's so cute!
I love going to sleep with my windows open so I can hear them, but sometimes, I just have to close the window 'cause they are sooooooo loud!
They are pretty amazing little creatures. I think they are attracted to my backyard because I have several little water gardens. They deposit their eggs in the water.
They are so sensitive to the presence of water, that if I go out to just water a few plants, they start singing! I don't know if they see the water, or just hear or smell it!
Ain't nature grand? :)

Occasionally Northern Mockingbirds will mimic frogs mixed in with their repertoires and often late at night into the early morning hours. Males will sing back and forth at each other while their respective mates are sitting on nests - thereby establishing their territorial boundaries more firmly. Other birds that mimic frogs are European Starlings - haven't yet heard them calling at night, but it's not uncommon for them to. Heard one the other day mimicking a Western Meadowlark.

I live in the Phoenix area of Arizona. Last evening about 6:00 pm, we were walking around a small lake near our home, when a flock of about 20 or so birds flew in and landed in two trees near us. They honestly sounded like bullfrogs. They were about the size of a duck, but more slender bodies and long necks. Their wings upon folding had a definite bend to them similar to what you might see in prehistoric birds (if that makes sense). My husband thought they were frogs and kept looking at the lake, but we definitely saw and heard the birds in the trees. Any ideas? I am stumped.

Sha7260, they may be bitterns. In my woods they sound like frogs. Very gutteral sound. They are hard to spot but fit your description. Google it and see if that's it.

Well, I heard a very loud croaking but assumed it had to be a bird as it is January and we have over 5 feet of snow on the ground. Never heard a frog in the winter before!

The American Woodcock sounds like a frog. You can listen to it online.

From PCB, Fl... without question, these are birds that hang out around the street lights at and around my home. I see and hear them nightly. They appear to be white, (however, he human eye sees in B&W at night), medium sized (about the size of a large robin), and quick. They make a load sound like a green tree frog. Any definite I.D. out there or just speculation???

I have been hearing a bird under my air conditioner in my tall old farmhouse. I hear it day and night. It started around March. It calls out in a harsh frog sound but only a few calls at a time. I haven't turned a/c on yet. I believed it was probably a ugly duckling type that had been abandoned because it's the only one.

Then last week I went to visit grandchildren and heard that same sound and again it was only 1 calling out. Is this possibly a new breed ? Ive lived here for 27 yrs an always got birds in upper window a/c's but not like this.

HU, it probably IS a frog. I have them all around my house including one in the air conditioner and one in the grill on my deck. It comes back there every spring.